1. Field of the Invention
The present invention relates to a monitoring device for distinguishing the operating state of an electrical load, and more particularly to such a devie which employs a monitor which is controllable by two threshold switches, the monitor being employed to output a first status report given a control potential supplied to the threshold switches, the control potential lying between a prescribed minimum value and a prescribed maximum value, and to output a second status report when these control potential limiting values are exceeded or fallen below.
2. Description of the Prior Art
For monitoring the operational status of electrical loads, for example, of the signal lamps of a light signal, it is not only known to identify the respective operating state of the electrical load by monitors connected in the current path of the load and to report the states to, for example, a central control location, but, rather, it is also known to check the proper operational behavior of the monitors. To this end, in a known remote control system (South African Letters Pat. No. 69/1938), the monitors are temporarily driven into one switching state and then into another switching state independently of the switching state of the electrical load monitored thereby. The receipt of the status reports for the monitors transmitted to the control location are converted at the control location into signals distinguishing the proper operational behavior of the monitors; given failure of the expected status reports, a failure report is triggered in order to initiate prescribed switching routines.
All in all, it is not sufficient to know whether an electrical load is traversed by current or not; in order to assure a specific operating behavior of the electrical load, a clear statement is necessary whether or not the current flowing through the load lies within prescribed limits. The load current at the connected load dare not lie below a prescribed threshold value, since the load cannot operate properly under such a condition; however, the load current also dare not exceed an upper value because the load can then be damaged. Moreover, the flow of a current lying above a prescribed threshold value can also be an indication that the measured load current is not flowing through the load but, rather, across a low-resistance bypass of the load.
A device for monitoring the upper and lower limiting values of a constant voltage level is known from U.S. Pat. No. 3,159,772, with which it is possible to identify whether a constant voltage to be monitored lies within a prescribed voltage range or deviates to an inadmissible degree from a nominal voltage. The device consist of two threshold switches to whose control inputs the constant voltage to be monitored is supplied; the response levels of the two threshold switches are determined by the prescribed minimum value and the prescribed maximum value of the constant voltage to be monitored. The threshold switches control a monitor connected thereto in such a manner that the monitor assumes one switching state given a constant voltage within the prescribed voltage range and assumes another switching state given a constant voltage beyond the prescribed voltage range.
The monitor of this known device can be checked as to operability during operation by employing techniques from South African Letters Pat. No. 69/1938, in that it is driven into its two possible operating states by supplying thereto separate check commands. The operability of the actual monitor can be tested by this check operation; the monitoring result, however, provides no information as to whether the switching structure by way of which the monitor is operationally controlled is, in turn, likewise operational, i.e. that the monitor in fact reacts in the desired manner to the control voltages of different magnitude.